Heat-producing compositions for use in self-heating packaging are known. Previous heat-producing compositions typically produce heat by combining two or more reagents in a manner which produces a reaction which creates heat. For example, a solid material may be combined with a liquid, such as water, to produce heat as a result of the hydration of the solid material. Alternatively, the liquid may be a solution containing a material that will react with the solid to produce an exothermic chemical reaction, thereby releasing heat. However, the prior heat-producing compositions exhibit certain undesirable characteristics.
U.S. Pat. No. 4,793,323 describes a self-heating container in which a multi-compartment package contains a food or liquid to be heated. The container includes two compartments that are separated by a membrane. Each of the compartments contains a reagent, and a member is provided for breaking the membrane. Upon breaking the membrane, the reagents (for example water and calcium chloride or calcium oxide) are allowed to mix and the resulting exothermic reaction heats the package and its contents. Among the disadvantages of the system is that the respective heat outputs of the disclosed reagent systems are such that a relatively large amount of such reagents is required to heat a relatively small amount of food or beverage.
U.S. Pat. No. 5,611,329 describes a flameless heater that consists of non-woven, water permeable polyester sheets that are thermally bonded together to form a number of pockets. Each of the pockets formed by the bonded polyester sheets is filled with a powder mixture of a Mg—Fe alloy, NaCl, antifoaming agents and an inert filler. Upon exposure to water, the Mg—Fe alloy under goes an exothermic reaction, that is, hydration of the magnesium component of the alloy. The heat generated by the hydration reaction can be used to, for example, heat military field rations. One disadvantage of the flameless heater is that the hydration of magnesium generates hydrogen gas, which may pose a safety hazard.
U.S. Pat. Nos. 5,935,486 and 6,248,257 describe portable heat sources that include a heat-producing composition comprising a mixture of an acidic anhydride or salt, a basic anhydride or salt and inert materials. Addition of water to the mixture results in exothermic reactions that generate heat. While the heating compositions provide some improved performance over prior systems, the disclosed systems provide only rough control over the rate of heat generation, and can result in compositions in which uncontrolled reactions occur, leading to rapid and potentially dangerous rates of heat generation. In addition the heat-producing compositions may, in some embodiments, generate caustic wastes that may complicate disposal of used heat sources.
It would be useful to overcome these and other shortcomings of the prior art in order to provide a heat-producing composition that provides a high heat output, in a controllable manner, without generating serious safety or environmental risks.